Beamforming provides signal-to-noise enhancement by combining the individual responses of a plurality of sensors in such a manner that the composite response is maximized in a specific direction, thus providing spatial filtering. This is achieved by compensating for the variations in the signal wavefront arrival time at the various sensors. The variations in the wavefront arrival time from a specific direction are equalized by introducing time delays to the sensor outputs. In addition, a shading (weighting) function is usually applied to the sensor responses to suppress grating lobes, local maxima in the composite response, inherent in the spatial filter.
Time delays can be generated by various techniques. The conventional analog techniques approximate time delays using Bessel filters or allpass networks. The major drawback to these circuits is performance degradation due to component value tolerances and variations with age and over the environmental conditions. The preferred approach is true time delay generation employing shift registers which provide accurate, repeatable delays. The signal is sampled and loaded into a First-In/First-Out (FIFO) shift register. Each time a new sample is entered, all the samples currently residing in the register move down one stage, and the sample at the far end is "pushed out." A register of length N sampling and shifting a signal at rate f.sub.s delays the signal by N/f.sub.s. In general, a beamformer with numerous sensor inputs requires a different delay for each input. (In some cases, symmetry conditions reduce the required number of different delays). The time delay produced by the shift register, N/f.sub.s, can be changed by varying either the register length, N, or the sample/shift rate, f.sub.s. However, in a sampled data system, it is desirable to use a single sample rate to avoid aliasing of clock artifacts. This is particularly important when large dynamic range is required since the aliased clock artifacts can be large compared to low level input signals. It is known that the use of multiple sample rates can cause in-band contamination even when they are synchronized by a master clock, unless specific, restrictive conditions are met. Therefore, the various delays are generated by employing shift registers of different lengths, all clocked at the same rate. This is depicted in FIG. 1, where 37 parallel registers 11 of a differing number of stages are simultaneously clocked at the same 980 KHz rate to provide a beamformer 10 (the summing circuit is not shown) for one configuration of sensors 12 and for one beam direction.
Although the time delays generated by a set of parallel FIFO shift registers are extremely accurate and repeatable, they may not be precise. The time delay through a shift register is a multiple of the sample/shift period; in other words, time is quantized by the sampling process. Therefore, given an arbitrary desired time delay, the closest integer multiple of the sample/shift period must be chosen, which means that the time delay error can be as large as .+-.1/2 f.sub.s. Deviations from "ideal" time delays degrade the beam pattern, especially by increasing the magnitude of the grating lobes. Hence, it is often necessary to sample/shift the signal at a rate much higher than the Nyquist frequency to reduce the time delay quantization error. If long time delays are required to form the beam, then the shift register length can become prohibitively long.